Metasomatic oxidation of upper mantle periodotite   总被引：1，自引：0，他引：1  

Anne V. McGuire  M. Darby Dyar  Jane E. Nielson 《Contributions to Mineralogy and Petrology》1991,109(2):252-264

Examination of Fe³⁺ in metasomatized spinel peridotite xenoliths reveals new information about metasomatic redox processes. Composite xenoliths from Dish Hill, California possess remnants of magmatic dikes which were the sources of the silicate fluids responsible for metasomatism of the peridotite part of the same xenoliths. Mössbauer spectra of mineral separates taken at several distances from the dike remnants provide data on Fe³⁺ contents of minerals in the metasomatized peridotite. Clinopyroxenes contain 33% of total iron (Fe_T) as Fe³⁺ (Fe³⁺/Fe_T=0.33); orthopyroxenes contain 0.06–0.09 Fe³⁺/Fe_T; spinels contain 0.30–0.40 Fe³⁺/Fe_T; olivines contain 0.01–0.06 Fe³⁺/Fe_T; and metasomatic amphibole in the peridotite contains 0.85–0.90 Fe³⁺/Fe_T. In each mineral, Fe³⁺ and Fe²⁺ cations per formula unit (p.f.u.) decrease with distance from the dike, but the Fe³⁺/Fe_T ratios of each mineral do not vary. Clinopyroxene, spinel, and olivine Fe³⁺/Fe_T ratios are significantly higher than in unmetasomatized spinel peridotites. Metasomatic changes in Fe³⁺/Fe_T ratios in each mineral are controlled by the oxygen fugacity of the system, but the mechanism by which each phase accommodates this ratio is affected by crystal chemistry, kinetics, rock mode, fluid composition, fluid/rock ratio, and fluid-mineral partition coefficients. Ratio increases in pyroxene and spinel occur by exchange reactions involving diffusion of Fe³⁺ into existing mineral grains rather than by oxidation of existing Fe²⁺ in peridotite mineral grains. The very high Fe³⁺/Fe_T ratio in the metasomatic amphibole may be a function of the high Fe³⁺/Fe_T of the metasomatic fluid, crystal chemical limitations on the amount of Fe³⁺ that could be accommodated by the pyroxene, spinel, and olivine of the peridotite, and the ability of the amphibole structure to accommodate large amounts of 3 + valence cations. In the samples studied, metasomatic amphibole accounts for half of the bulk-rock Fe₂O₃. This suggests that patent metasomatism may produce a greater change in the redox state of mantle peridotite than cryptic metasomatism. Comparison of the metasomatized samples with unmetasomatized peridotites reveals that both Fe²⁺ and Fe³⁺ cations p.f.u. were increased during metasomatism and 50% or more of iron added was Fe³⁺. With increasing distance from the dike, the ratio of added Fe³⁺ to added Fe²⁺ increases. The high Fe³⁺/Fe_T of amphibole and phlogopite in the dikes and in the peridotite, and the high ratios of added Fe³⁺/added Fe²⁺ in pyroxenes and spinel suggest that the Fe³⁺/Fe_T ratio of the metasomatic silicate fluid was high. As the fluid perolated through and reacted with the peridotite, Fe³⁺ and C–O–H volatile species were concentrated in the fluid, increasing the fluid Fe³⁺/Fe_T.  相似文献   

A study of the polymorphic transformations in Co2SiO4     

Anne R. Remsberg  Robert C. Liebermann 《Physics and Chemistry of Minerals》1991,18(3):161-170

The olivine-beta phase transformation in Co₂SiO₄ has been studied in a large-volume high-pressure apparatus (USSA-2000). The experimental conditions straddle the univariant phase boundaries due to the presence of a substantial temperature gradient (150–200° C over the 3-mm length of the sample). At conditions close to equilibrium, the transformation mechanism is one of limited nucleation with rapid growth of large crystals of the beta phase. Transmission electron microscopy (TEM) analysis shows that the cation stacking faults in Co₂SiO₄-spinel are identical to those seen in numerous spinels. The 010 faults in beta-Co₂SiO₄ are found to be identical to those seen in beta-(Mg,Fe)₂SiO₄ found in shocked veins in meteorites.  相似文献   

Book reviews     

H. Freitag  Mary Anne Kunkel  Y. H. Fan 《GeoJournal》1988,17(2):300-306

  相似文献   

Arrested charnockite formation in southern India and Sri Lanka   总被引：7，自引：3，他引：7  

E. C. Hansen  A. S. Janardhan  R. C. Newton  W. K. B. N. Prame  G. R. Ravindra Kumar 《Contributions to Mineralogy and Petrology》1987,96(2):225-244

Arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high-grade terrains of southern India and Sri Lanka. Two major kinds of orthopyroxene-producing reactions are recognized. Breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic/granitic gray gneiss produced the regional orthopyroxene isograd, manifest in charnockitic mottling and veining of mixed-facies exposures, as at Kabbal, Karnataka, and in the Kurunegala District of the Sri Lanka Central Highlands. Chemical and modal analyses of carefully chosen immediately-adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of CaO, MgO and FeO and gains of SiO₂ and Na₂O. Rb and Y are depleted in the charnockite. Another kind of charnockitization is found in paragneisses throughout the southern high-grade area, and involves the reaction of biotite and quartz±garnet to produce orthopyroxene and K-feldspar. Although charnockite formation along shears and other deformation zones at such localities as Ponmudi, Kerala is highly reminiscent of Kabbal, close pair analyses are not as suggestive of open-system behavior. This type of charnockite formation is found in granulite facies areas where no prograde amphibole-bearing gneisses exist and connotes a higher-grade reaction than that of the orthopyroxene isograd. Metamorphic conditions of both Kabbaltype and Ponmudi-type localities were 700°–800° C and 5–6 kbar. Lower P(H₂O) in the Ponmudi-type metamorphism was probably the definitive factor.CO₂-rich fluid inclusions in quartz from the Kabbaltype localities support the concept that this type of charnockite formation was driven by influx of CO₂ from some deep-seated source. The open-system behavior and high oxidation states of the metamorphism are in accord with the CO₂-streaming hypothesis. CO₂-rich inclusions in graphitebearing charnockites of the Ponmudi type, however, commonly have low densities and compositions not predictable by vapor-mineral equilibrium calculations. These inclusions may have suffered post-metamorphic H₂ leakage or some systematic contamination.Neither the close-pair analyses nor the fluid inclusions strongly suggest an influx of CO₂ drove charnockite formation of the Ponmudi type. The possibility remains that orthopyroxene and CO₂-rich fluids were produced by reaction of biotite with graphite without intervention of fluids of external origin. Further evidence, such as oxygen isotopes, is necessary to test the CO₂-streaming hypothesis for the Ponmudi-type localities.  相似文献   

Textural evolution of synthetic anhydrite-halite mylonites     

John V. Ross  Stephen J. Bauer  Francis D. Hansen   《Tectonophysics》1987,140(2-4):307-326

The rheology and textural evolution of mylonites for both crustal and mantle rocks are generally poorly understood. Stress and strain partitioning between shear zones and wall-rock, foliation development, flow mechanisms, and constitutive relations are little known. To address these problems, we have begun an experimental study in which anhydrite and halite aggregates are deformed in simple shear. Homogeneous mixes at intervals of 10 vol.% of anhydrite and halite powders of 90 to 180-μ m grain-size comprise the 1-mm-thick zone between the 35 °-precut surfaces in 2 by 4 cm right-circular cylinders of anhydrite. All specimens are confined under 200 MPa (P_c) and tested at 300 °C (T), producing an aggregate of near-zero porosity. All specimens are shortened at an axial displacement rate of 5×l0⁻⁶ cm/s to shear strains of about 2.0. Most stress-strain curves have a similar form. They are characterized by an initial linear response that is followed by a work-hardening region that leads to a plateau of constant stress whilst strain continues to accumulate. This apparent steady-state behavior is followed by a drop in stress to a second apparent steady-state stress. The strain at this stress drop and the magnitude of the drop increase with increasing anhydrite content. The second (lower) steady-state stress for all mixes approaches that of sheared pure halite. Optical examination of the fabrics shows that straining under the initial apparent steady-state is accommodated by both mineral phases. A foliation evolves as defined by mineral segregation and grain elongation of halite at 15 ° to 30 ° to the shear-zone walls. Anhydrite grains rotate until their (001) 010 slip system parallels the foliation. The anhydrite also exhibits twin lamellae, kinks and microfractures. Following the stress drop, rheologic behavior is dominated by dynamic recovery in halite, the recrystallized grain size increasing, consistent with the lower stress level.  相似文献   

Barium content of marine sediments near drilling sites: A potential pollutant indicator     

Tsaihwa J. Chow  John L. Earl  John H. Reed  Nicholas Hansen  Victor Orphan 《Marine pollution bulletin》1978,9(4):97-99

Barite is commonly used as a weighting agent in oil- and gas-well drilling mixture; therefore, the distribution pattern of barium in marine sediments near drilling sites may provide an indicator of anthropogenic chemical contamination from these operations. Present average content of barium in benthic sediments from the Southern California Bight was determined to be 637 ppm, with a range from 43 to 1899 ppm, and that of the mainland and the channel island intertidal sediments averaged 879 and 388 ppm, respectively.  相似文献   

Indications of a biological and biochemical evolutionary trend during the archean and early proterozoic     

Bartholomew Nagy  Lois Anne Nagy  John E. Zumberge  Deborah S. Sklarew  Phillip Anderson 《Precambrian Research》1977,5(2):109-120

Micropaleontological observations and organic geochemical analyses imply, but certainly do not prove, that life may not have been present ～3,800 Ma ago in Southwestern Greenland; however, physiologically complex prokaryotes flourished by the Early Proterozoic in other locations. It is, of course, also possible that life may have appeared earlier in locations other than Greenland. Investigations of Precambrian biological and biochemical evolutionary trends require interdisciplinary efforts, up-to-date instrumentation and methodology (such as ultramicrochemical analyses of individual microfossils/microstructures), and caution in the interpretation of experimental results.  相似文献   

The transformation of amphibolite facies gneiss to charnockite in southern Karnataka and northern Tamil Nadu,India   总被引：4，自引：0，他引：4  

A. S. Janardhan  R. C. Newton  E. C. Hansen 《Contributions to Mineralogy and Petrology》1982,79(2):130-149

Amphibolite facies metamorphic grade gives way southward to the granulite grade in southern Karnataka, as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes. Petrologic investigations in the transitional zone south of Mysore have established the following points:

1. The transition is prograde. Amphibole-bearing gneisses intimately associated with charnockite at Kabbal and several similar localities are not retrogressive after charnockite, as proved by patchy obliteration of their foliation by transgressive, very coarse-grained charnockite, high fluorine content of biotite and amphibole in gneisses, and high large-ion lithophile element contents in gneisses and charnockites. These features are in contrast to very low fluorine in retrogressive amphiboles and biotites, very low large-ion lithophile element contents, and zonal bleaching of charnockite, in clearly retrogressive areas, as at Bhavani Sagar, Tamil Nadu.
2. Metamorphic temperatures in the transitional areas were 700°–800° C, pressures were 5–7 kbar, and H₂O pressures were 0.1–0.3 times total pressures, based on thermodynamic calculations using mineral analyses. Dense CO₂-rich fluid inclusions in the Kabbal rocks confirm the low H₂O pressures at the first appearance of orthopyroxene. Farther to the south, in the Nilgiri Hills and adjacent granulite massif areas, peak metamorphic temperatures were 800°–900° C, pressures were 7–9 kbar, and water pressures were very low, so that primary biotites and amphiboles (those with high F contents) are rare.
3. The incipient granulite-grade metamorphism of the transitional areas was introduced by a wave of anatexis and K-metasomatism. This process was arrested by drying out under heavy CO₂ influx. Charnockites so formed are hybrids of anatectic granite and metabasite, of metabasite and immediately adjacent gneiss, or are virtually isochemical with pre-existing gneiss despite gross recrystallization to granulite mineralogy. These features show that partial melting and metasomatism are attendant, rather than causative, in charnockite development. Copious CO₂ from a deep-crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis. Granulite metamorphism of both neosome and paleosome followed. The process is very similar to that deduced for the Madras granulites by Weaver (1980). The massif charnockites, for the most part extremely depleted in lithophile minor elements, show many evidences of having gone through the same process.

A major problem remaining to be solved is the origin of the large amount of CO₂ needed to charnockitize significant portions of the crust. The most important possibilities include CO₂ from carbonate minerals in a mantle “hot spot” or diapir, from emanations from a crystallizing basaltic underplate, or from shelf sediments trapped at the continent-continent interface in continental overthrusting. Ancient granulite massifs may be such suture zones of continental convergence.  相似文献   

Pyrite and oxidized iron mineral phases formed from pyrite oxidation in salt marsh and estuarine sediments     

George W. Luther  Anne Giblin  Robert W. Howarth  Robert A. Ryans 《Geochimica et cosmochimica acta》1982,46(12):2665-2669

We used scanning electron microscopy and energy dispersive X-ray analysis to examine sediments from vegetated portions of three salt marshes, the Great Sippewissett Marsh (Cape Cod, MA), Sapelo Island (Georgia), and the Hackensack Meadowlands (N.J.), and from the sediments of an estuary, Newark Bay (N.J.). Pyrite particles were abundant in sediments from all sites. Both fine grained pyrite crystals and framboids were found. Single, fine grained crystals (diameter = 0.2 to 2.0 micrometers) predominated in all samples, strong evidence for rapid formation of pyrite.We also found both microcrystalline and framboidal iron-oxyhydroxide phases in many of the sediment samples. This is evidence of pyrite oxidation within the sediments and suggests that iron is conserved in salt marshes even as pyrite is oxidized. The thermodynamic stability of iron phases in marsh sediments, and recent pyrite oxidation studies in coal, suggest goethite as the crystalline iron-oxyhydroxide phase present. In addition, we sometimes found a red amorphous coating on grass roots from the Great Sippewissett and Sapelo Island marshes. This coating is likely a form of hydrated iron (III) oxide.  相似文献   

Observational study of terrestrial eigenvibrations     

Roger A. Hansen 《Physics of the Earth and Planetary Interiors》1982,28(1):27-69

A comprehensive analysis has been made of analog and digital recordings of eigenvibration ground motion obtained following four great earthquakes; August 1976 (Philippines), August 1977 (Indonesia), September 1979 (West Irian), and December 1979 (Colombia). The time series (ranging in length from ～28 to ～140 h) are assumed to be linear combinations of damped harmonics in the presence of noise. Tables are calculated from values of the four parameters: Θ, used in describing eigenvibrations, period of oscillation, amplitude, damping factor Q, and phase together with their statistical uncertainties (53 spheroidal modes, ${}_0\text{S}{}_4\text{to}{}_0\text{S}{}_{48}$, and 13 torsional modes, ${}_0\text{T}{}_8\text{to}{}_0\text{T}{}_{45}$). The estimation procedures are by the methods of complex demodulation and non-linear regression that specifically incorporate into the basic model the decaying aspect of the oscillations. These methods, extended to simultaneous estimations of groups of modes, help to eliminate measurement error and measurement bias from estimations of Θ. The result is that overtone modes very near in frequency to fundamental modes can, under certain conditions, be resolved through a non-linear regression technique, although parameter uncertainties are underestimated in general.Of the time series analyzed, 17 were from a northern California regional network of ultra-long period seismographs at Berkeley (three components), Jamestown (vertical component), and Whiskeytown (vertical component) following the four listed earthquakes. The other 7 time series were recorded digitally by the worldwide IDA network following the 1977 Indonesian earthquake. Weighted regional and worldwide averages were made for period and Q of each eigenvibration mode.From the theoretical viewpoint, comparisons of measured period, Q, amplitude, and phase for all modes analyzed led to five conclusions. First, there are no detectable systematic shifts in period, Q, or phase of eigenvibrations within a region whose dimensions are less than a wavelength. Second, though not conclusive, there may be slight systematic shifts in period (<0.65 s) and relative amplitudes within the California regional network due to different source positions and mechanisms. Differences in Q values are not statistically significant. Third, even though differences in period obtained worldwide were as great as 1.33 s (≈0.33%), differences between Q values (as great as 20%) for the same mode were not significant. The conclusion is that the damping characteristics of singlet eigenfunctions are not observed to be significantly different. Fourth, the assumption that a multiplet ${}_{\mathrm{n}}\text{S}{}_{\mathrm{l}}$ behaves as a single oscillation is valid from at least ${}_0\text{S}{}_7$ through ${}_0\text{S}{}_{30}$. Fifth, no systematic pattern emerged for the shift of eigenperiod as a function of order / or posit on the Earth.  相似文献